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DSO Samplerates - From when they´re "enough" and why?
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nctnico:

--- Quote from: Fungus on January 18, 2023, 11:58:51 am ---
--- Quote from: magic on January 18, 2023, 09:51:34 am ---AFAIK they have switchable digital filters.

--- End quote ---

The RigolDS1054Z has capacitors in the analog front end that are switched in using MOSFETs.

--- End quote ---
That only gives you a 1st order filter. Good for getting a nicer roll-off but not to prevent aliasing.

edit: typo
Fungus:

--- Quote from: nctnico on January 18, 2023, 12:19:19 pm ---That only gives you a 1st filter. Good for getting a nicer roll-off but not to prevent aliasing.

--- End quote ---

Sure, but it's already in there and it's free. It would be nice to use it.
vk6zgo:
Very early DSOs had abysmal memories, so for long settings of time/div, the sample rate was reduced, as there was no place to save the number of samples that would result from sampling, & displaying say, 2 cycles of a 50Hz sine wave, using the maximum sample rate, so the sample rate was reduced.

A sine wave of that order could be easily accommodated, using around 1kSa/s, but complex signals are "another Kettle of fish"
Analog video signals are particularly nasty, with components out to 5MHz, or sometimes more.

Not only are they wideband, but the component frequencies of that bandwidth are also constantly changing.
TV people were usually very enthusiastic to see their first DSO, but all their hopes were dashed by the unusable displays presented, as the instrument could not represent the real waveform.

All the discussion so far seems to be drifting into the problems of higher frequencies, where anti-aliasing filters come into play, but if sampling rates are still reduced at much lower frequencies, any alias components will still be present for those lower frequencies which 'scopes are very commonly required to observe,
nctnico:

--- Quote from: vk6zgo on January 18, 2023, 01:27:56 pm ---All the discussion so far seems to be drifting into the problems of higher frequencies, where anti-aliasing filters come into play, but if sampling rates are still reduced at much lower frequencies, any alias components will still be present for those lower frequencies which 'scopes are very commonly required to observe,

--- End quote ---
That is why a good DSO has peak detect. The ADCs run at full sample rate and the min/max of the sample interval are stored.

At one of my employers we had a brand new 500MHz Lecroy DSO sitting on top of a cabinet. Looked like a really nice oscilloscope but nobody ever used it. The lack of peak detect made it completely useless for the work we where doing. I don't think I have ever seen it being switched on. When I visited a couple of years later (after I left), it was still sitting there.
alm:

--- Quote from: vk6zgo on January 18, 2023, 01:27:56 pm ---All the discussion so far seems to be drifting into the problems of higher frequencies, where anti-aliasing filters come into play, but if sampling rates are still reduced at much lower frequencies, any alias components will still be present for those lower frequencies which 'scopes are very commonly required to observe,

--- End quote ---
Unless you are zooming in a lot after acquisition, the "resampling" by the display's limited horizontal resolution, which unless you're using an ancient Tek DSO is many orders of magnitude lower than the sample memory, is probably a bigger worry. But I believe most scopes, including Lecroy scopes, do some sort of peak detect when they resample data for display?


--- Quote from: nctnico on January 18, 2023, 01:40:10 pm ---That is why a good DSO has peak detect. The ADCs run at full sample rate and the min/max of the sample interval are stored.

--- End quote ---
Having enough sample memory so the sample resolution is much higher than the display resolution also helps a lot. I used to use peak detect all the time back when my (Tek TDS-220) scope had 2.5 kS of sample memory, and so it was constantly under-sampling. On modern deep memory scopes I can easily get by without, since sample rates are generally fast enough to capture the data I need. Except in extreme cases where the duty cycle is approaching the sample memory depth, like a 1ns pulse with a repetition rate of 1s, which would need multiple GS of memory depth to properly sample. But this is not something I encounter often, plus if I only care about the pulse timing, I might well be using a logic analyzer instead.
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